Mid-infrared emission properties of erbium-doped fluorite-type crystals

We report on a comparative study of the spectroscopic properties and mid-infrared laser performance of five 5 at.% Er 3+ -doped fluorite-type crystals MF 2 , including parent compounds CaF 2 , SrF 2 , BaF 2 , and solid-solution (“mixed”) ones (Ca,Sr)F 2 and (Sr,Ba)F 2 . In the M = Ca → Sr → Ba series, the host matrix phonon energy decreases, the absorption and mid-infrared emission spectra of Er 3+ become narrower and more structured, and the luminescence lifetimes of the 4 I 11/2 and 4 I 13/2 Er 3+ manifolds increase. The Er 3+ transition probabilities were calculated using the Judd-Ofelt theory. In the “mixed” compounds, the Er 3+ ions tend to reside in the larger / heavier cation environment. The low-temperature (12 K) spectroscopy evidences the presence of a single type of clusters at this doping level; the crystal-field splitting for Er 3+ ions in clusters was determined. Continuous-wave low-threshold laser operation at ∼2.8 µm (the 4 I 11/2 → 4 I 13/2 transition) was achieved with all five Er 3+ :MF 2 crystals. The maximum achieved laser slope efficiency was 37.9% (Er 3+ :CaF 2 ), 23.5% (Er 3+ :SrF 2 ) and 17.2% (Er 3+ :BaF 2 ).